High-performance Blue Organic Light-emitting Diodes Research Articles

Efficient Deep-Blue Organic Light-Emitting Diodes Employing Doublet Sensitization.

Fast and efficient exciton utilization is a crucial solution and highly desirable for achieving high-performance blue organic light-emitting diodes (OLEDs). However, the rate and efficiency of exciton utilization in traditional OLEDs, which employ fully closed-shell materials as emitters, are inevitably limited by spin statistical limitations and transition prohibition. Herein, a new sensitization strategy, namely doublet-sensitized fluorescence (DSF), is proposed to realize high-performance deep-blue electroluminescence. In the DSF-OLED, a doublet-emitting cerium(III) complex, Ce-2, is utilized as sensitizer for multi-resonance thermally activated delayed fluorescence emitter ν-DABNA. Experimental results reveal that holes and electrons predominantly recombine on Ce-2 to form doublet excitons, which subsequently transfer energy to the singlet state of ν-DABNA via exceptionally fast (over 108s-1) and efficient (≈100%) Förster resonance energy transfer for deep-blue emission. Due to the circumvention of spin-flip in the DSF mechanism, near-unit exciton utilization efficiency and remarkably short exciton residence time of 1.36µs are achieved in the proof-of-concept deep-blue DSF-OLED, which achieves a Commission Internationale de l'Eclairage coordinate of (0.13, 0.14), a high external quantum efficiency of 30.0%, and small efficiency roll-off of 14.7% at a luminance of 1000cdm-2. The DSF device exhibits significantly improved operational stability compared with unsensitized reference device.

Medium-Ring Strategy Enables Multiple Resonance Emitters with Twisted Geometry and Fast Spin-Flip to Suppress Efficiency Roll-Off.

Suppressing aggregation-caused quenching (ACQ) effect and reducing device efficiency roll-off are both crucial yet challenging for multi-resonance (MR) emitters. Herein, we put forward a medium-ring strategy to design efficient MR emitters that feature heptagonal tribenzo[b,d,f]azepine (TBA) donors. The highly twisted conformation enlarges the intermolecular distances between the MR-emitting cores, and thus suppresses ACQ effect. Meanwhile, the introduction of heptagonal donors enhances spin-orbital coupling, so as to accelerate reverse intersystem crossing (RISC) process. This medium-ring strategy gives rise to the first example of blue MR emitter that simultaneously possesses radiative decay rate as fast as 108 s-1 and RISC rate as fast as 106 s-1 . Accordingly, DTBA-B2N3 enables to assemble high-performance blue organic light-emitting diodes (OLEDs) with maximum external quantum efficiency (EQEmax ) of 30.9 % and alleviated efficiency roll-off (EQE1000 : 20.5 %).

Investigation of the localized surface plasmon resonance of Ag@SiO2 core-shell nanocubes and its application in high-performance blue organic light-emitting diodes

A blue organic light-emitting diode (OLED) with silica-coated silver nanocubes (Ag@SiO2 NCs) inserted at the hole transporting layer/emission layer interface is reported. The localized surface plasmon resonance (LSPR) properties of the Ag@SiO2 NCs were characterized by the measured absorption spectra, stable-stated and transient photoluminescence, and calculated dipole radiation power. The results suggest that the Ag NCs significantly improved the radiation intensity of the nearby excitons due to their sharp corners and edges, but had less impact on the radiation rate of the excitons. The exciton recombination zone in the blue OLED was confirmed by a group of devices with an ultra-thin yellow emission layer located at different places, which helped to figure out the distribution of the excitons around the Ag@SiO2 NCs and deeply understand the coupling between the excitons and the Ag@SiO2 NCs. In our blue OLED, an appropriate distance between the Ag NCs and the excitons was realized by the SiO2 coating layer and the exciton distribution, which greatly improved the energy transfer between the excitons and the Ag NCs. In addition, the LSPR enhanced electric field around the Ag@SiO2 NCs improved the carrier injection at the hole transporting layer/emission layer interface and increased the current density of the blue OLED. Finally, the blue OLED with a simple triple layer structure achieved a high current efficiency of 51.1 cd A−1.

Improved electron injection and efficiency in blue organic light-emitting diodes using coupled electric field near cathode

Abstract In this paper, we report high-performance blue organic light-emitting diodes (OLEDs) based on evaporated silver nanoclusters (SNCs) located near the aluminum cathode. We investigate the influence of localized surface plasmon resonance (LSPR) and trapping effects of SNCs on the performances of blue OLEDs depending on the size, distribution, and location of the SNCs. The research results reveal when the SNCs are located in the electron transporting layer, the serious trapping effect would cause reduced current density and efficiency of our blue OLEDs. Fortunately, when the SNCs are close to the cathode, owing to the electric field coupling effect between the LSPR of SNCs and the surface polarization of aluminum cathode, the resonant electric field is concentrated near the cathode, thereby enhancing the electron injection. The balanced carrier injection and transport processes improve the performance of blue OLEDs. The optimized device with SNCs located 2 nm away from the aluminum cathode achieves the highest current efficiency of 40.7 cd/A, enhancing by 15% compared to the control device without SNCs.

High-performance blue organic light-emitting diodes with 20% external electroluminescence quantum efficiency based on pyrimidine-containing thermally activated delayed fluorescence emitters

High-performance pure blue thermally activated delayed fluorescence (TADF) emitters consisting of pyrimidine and diphenylacridan units are developed.

Realization of high-performance blue organic light-emitting diodes using multi-emissive layers

High-performance blue organic light-emitting diodes (OLEDs) are developed. A concept of using multiple-emissive layer (EML) configuration is adopted. In this letter, bis(2-methyl-8-quinolinolate)-4(phenylphenolato)Al (BAlq) and 9,10-di(naphtha-2-yl)anthracene (ADN), which serve n- and p-type EMLs, respectively, are used to evaluate and demonstrate the multi-EML concept for blue OLEDs. The thickness effect of individual EMLs and the number of EMLs, e.g., triple and quadruple EML components, on the power efficiency of blue OLEDs are systematically investigated. To illustrate the point, the total thickness of the emissive region in different blue OLEDs are kept contact at 30 nm for comparison. The power efficiency of blue OLEDs with a quadruple EML structure of BAlq/ADN/BAlq/ADN is about 40% higher than that of blue OLEDs having a single EML unit. The Commission Internationale deL’eclairage color coordinates of multi-EML OLEDs have values that represent the average of blue emissions from individual EMLs of BAlq and ADN. OCIS codes: 230.0230, 230.3670, 230.0250, 160.4890, 130.5990. doi: 10.3788/COL201412.012302.

Phenyl-substituted fluorene-dimer cored anthracene derivatives: highly fluorescent and stable materials for high performance organic blue- and white-light-emitting diodes

A new series of highly fluorescent blue-emitting materials based on fluorene and anthracene hybrids are designed and synthesized for organic light-emitting diodes (OLEDs). These materials feature a phenyl-substituted fluorene dimer as a bulky and rigid core and anthracene as a functional active group. The novel use of a phenyl-substituted fluorene dimer as building skeleton to design functional molecules is reported for the first time. The thermal, photophysical, electrochemical, and electroluminescent (EL) properties are presented, as well as combined density functional study of their geometry and electronic structure. These compounds show excellent thermal resistance with high glass transition temperature (Tg) in the range 159–257 °C, thermal decomposition temperature (Td) 441–495 °C, and high fluorescent quantum yield (ΦF = 0.61–0.96, relative to 9,10-diphenylanthracene) as well as good film-forming and morphological stability. Remarkably, high-performance blue OLEDs are also fabricated in a simple three-layer device architecture using these compounds as emissive layer with luminance efficiency of 2.2–5.1 cd A−1 as a non-doped blue emitter and even higher efficiency of up to 13.6 cd A−1 and maximum external quantum efficiency 4.8% is obtained when doped a blue fluorescent dye, 4,4′-(1E,1′E)-2,2′(biphenyl-4,4′diyl)bis(ethane-2,1-diyl)bis(N,N-dip-tolyaniline) (DPAVBi). Furthermore, we fabricate highly efficient fluorescent white OLEDs employing an interesting emission in the longer wavelength of one of our compound combined with DPAVBi emission to achieve stable white light emission in a binary blend single emissive layer with high efficiency of 14.8 cd A−1 (5.3 lm W−1) and maximum brightness of 50248 cd m−2.

Optimization of high-performance blue organic light-emitting diodes containing tetraphenylsilane molecular glass materials.

Molecular glass material (4-(5-(4-(diphenylamino)phenyl)-2-oxadiazolyl)phenyl)triphenylsilane (Ph(3)Si(PhTPAOXD)) was used as the blue light-emitting material in the fabrication of high-performance organic light-emitting diodes (OLEDs). In the optimization of performance, five types of OLEDs were constructed from Ph(3)Si(PhTPAOXD): device I, ITO/NPB/Ph(3)Si(PhTPAOXD)/Alq(3)/Mg:Ag, where NPB and Alq(3) are 1,4-bis(1-naphylphenylamino)biphenyl and tris(8-hydroxyquinoline)aluminum, respectively; device II, ITO/NPB/Ph(3)Si(PhTPAOXD)/TPBI/Mg:Ag, where TPBI is 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; device III, ITO/Ph(2)Si(Ph(NPA)(2))(2)/Ph(3)Si(PhTPAOXD)/TPBI/Mg:Ag, where Ph(2)Si(Ph(NPA)(2))(2) is bis(3,5-bis(1-naphylphenylamino)phenyl)-diphenylsilane, a newly synthesized tetraphenylsilane-containing triarylamine as hole-transporting material; device IV, ITO/Ph(2)Si(Ph(NPA)(2))(2)/NPB/Ph(3)Si(PhTPAOXD)/TPBI/Mg:Ag; device V, ITO/CuPc/NPB /Ph(3)Si(PhTPAOXD)/Alq(3)/LiF/Al, where CuPc is Cu(II) phthalocyanine. Device performances, including blue color purity, electroluminescence (EL) intensity, current density, and efficiency, vary drastically by changing the device thickness (100-600 A of the light-emitting layer) and materials for hole-transporting layer (NPB and/or Ph(2)Si(Ph(NPA)(2))(2)) or electron-transporting material (Alq(3) or TPBI). One of the superior OLEDs is device IV, showing maximum EL near 19 000 cd/m(2) with relatively low current density of 674 mA/cm(2) (or near 3000 cd/m(2) at 100 mA/cm(2)) and high external quantum efficiency of 2.4% (1.1 lm/W or 3.1 cd/A). The device possesses good blue color purity with EL emission maximum (lambda(max)(EL)) at 460 nm, corresponding to (0.16, 0.18) of blue color chromaticity on CIE coordinates. In addition, the device is reasonably stable and sustains heating over 100 degrees C with no loss of luminance on the basis of the annealing data for device V. Formation of the exciplex at the interface of NPB and Ph(3)Si(PhTPAOXD) layers is verified by EL and photoluminescence (PL) spectra studies on the devices with a combination of different charge transporting materials. The EL due to the exciplex (lambda(max)(EL) at 490-510 nm) can be properly avoided by using a 200 A layer of Ph(3)Si(PhTPAOXD) in device I, which limits the charge-recombination zone away from the interface area.